Scientific Study

Nuts are an energy-dense food, yet regular consumption is not associated with weight gain. A proportion of the fats found within nuts remains encapsulated within cell walls and cannot be digested. Metabolizable energy can be explored by measuring fecal fat excretion in human studies, and fat release among in vitro studies. This systematic review with narrative synthesis aimed to examine the metabolizable energy of tree nuts and peanuts (PROSPERO CRD42021252287). PubMed, MEDLINE, CINAHL, Cochrane, and Embase databases were searched to June 2021. Both in vitro and human studies (adults ≥18 years) were included. Data was synthesised via narrative synthesis with results reported in summary tables and compared between form, processing, and dose of nuts, where available. Twenty-one studies were included. The metabolizable energy of nuts was consistently lower than that predicted by Atwater factors for investigated nut types (almonds, cashews, hazelnuts, pistachios, walnuts, and peanuts). The mechanisms may relate to a lower fat release from nuts, hence higher fecal fat excretion; however, this review did not consider the digestibility of carbohydrates and protein which should be considered when interpreting the outcomes. Metabolizable energy (ME) was influenced by nut type (ME = 22.6kJ/g for pistachios; ME = 18.5kJ/g for raw almonds), physical form (flour > chopped > whole nuts), heat processing (butter > roasted > raw) and dose of consumption. The lower-than-expected metabolizable energy may explain a lack of association between nut intake and body weight observed in the literature and has implications for the development of food composition databases, food labelling and informing dietary guidelines. However, the strength of the evidence base was reduced by the variation in methods used between studies, suggesting that further clinical trials are needed to determine the impact of the findings of this review for clinical dietetics. STATEMENT OF SIGNIFICANCE: This study is the first to systematically review the metabolizable energy content and lipid bioaccessibility of tree nuts and peanuts. The results of this study suggest that the metabolizable energy of nuts is lower than expected, due to a lower lipid release during processing and digestion, and is impacted by nut type, physical processing and heat treatment of nuts. https://doi.org/10.1016/j.advnut.2023.03.006